In recent years, due to environmental issues and resource issues, strong demands have been placed on tires for automobile use for improving low fuel consumption. At the same time, excellent wet grip has been sought due to safety concerns. Cross-linked rubber obtained by using a composition containing silica as a filler are superior to cross-linked rubber obtained by using a composition containing carbon black in low heat buildup, so the rolling resistance when used for a tire becomes smaller. For this reason, by using cross-linked rubber obtained by using a composition containing silica to make tires, it is possible to obtain tires excellent in low fuel consumption.
However, even if adding silica to conventional rubber, the affinity of rubber and silica is insufficient, so these easily separate. Due to this, the processability of the rubber composition before cross-linking is poor. Further, the cross-linked rubber obtained by cross-linking this becomes insufficient in low heat buildup.
Therefore, to improve the affinity of rubber and silica, for example, it has been proposed to add various silane coupling agents such as disclosed in Patent Document 1, and Patent Document 2 to the rubber composition. However, advanced processing techniques are required for handling silane coupling agents and silane coupling agents are expensive, so if the amounts added become greater, there is the problem that tires will become higher in manufacturing costs.
To solve this problem, for example, as disclosed in Patent Document 3, Patent Document 4, etc., when using the solution polymerization method to obtain a rubber polymer, the technique of causing a modifying agent to react with the active end of a polymer so as to impart affinity with silica to the rubber itself has been studied. However, due to the growing demand for low fuel consumption and wet grip on automobile tires in recent years, rubber able to give cross-linked rubber further excellent in low heat buildup and excellent in wet grip has been demanded.